This invention relates to a discharge lamp with arc extinguishing structure, and more particularly, to a discharge lamp which has an arc extinguishing structure coming into action at the end of life of the lamp.
The electron emission of the discharge tube is ensured by the electrode coating. At the end of lamp life, the electrode coating is used up, the emission capability decreases and the lamp voltage increases. As a consequence, the temperature of the electrode and the component parts in its vicinity significantly increases which leads to the breakage of parts, gas formation, melting of the plastic parts and burning-in of the lampholder. This harmful increase in temperature is avoided in a known way by releasing arc extinguishing gases from a structure located in the immediate vicinity of the electrode. The arc extinguishing gases have relatively low molecular weight but high ionization energy. So the lamp will extinguish and will not start any more. The component parts and objects in the vicinity of the lamp will not be endangered any longer.
An end of life arc extinguishing structure is described in U.S. Pat. No. 5,449,971. In this structure, glass beads containing CaCO3 are melted on the electrode lead wires, and CO2 is released on the effect of an increase in the temperature of the lead wire. Strontium carbonate (for higher temperatures) and barium carbonate may also be used. The decomposition temperature of CaCO3 is 525 to 550xc2x0 C. and the decomposition temperatures of SrCO3 and BaCO3 are higher. A disadvantage of this structure is that the glass bead must soften below this temperature. Therefore, a glass of special composition is necessary which also ensures that the arc extinguishing gas cannot be released during lamp life.
In the discharge lamp according to U.S. Pat. No. 5,585,693, a capsule filled with metal hydride powder is placed in the vicinity of the electrode. In one of the embodiments, a wire supporting the capsule is sealed into the glass of the pinched stem. In this case, however, the radiated heat is insufficient. In another embodiment, this wire is sealed to the current lead wires across an electrically insulating glass bead for a better heat transfer. The metal hydride used in the capsule is preferably titanium hydride, but zirconium or hafnium hydride or the alloy thereof may also be chosen which may be further alloyed with cobalt, iron, nickel, manganese, lanthanum or with the combination thereof.
The capsule may be made of steel or another metal alloy, and it is closed by crimping after filling the powder. Crimping, however, is not a gas-tight seal and so the hydrogen gas released from the metal hydride can get out from the capsule into the discharge space of the lamp. In this structure, the use of the capsule as a separate part is a disadvantage.
In the structure according to U.S. Pat. No. 5,705,887, a paste containing metal hydride is deposited on the current lead wire from which hydrogen is released in order to extinguish the discharge. The paste is applied to the surface of a glass bead also used as a support component preferably in two portions, so that no arc-over is caused between the lead wires. Fine titanium hydride powder is mixed to the paste. This structure has the disadvantage that the paste on the lead wire reaches the temperature at which the hydrogen is released from the titanium hydride with a significant delay only. In addition, titanium hydride decomposes at a higher temperature.
One of the greatest disadvantage of all known structures is that the temperature increases continuously rather than abruptly at the end of life due to their higher thermal inertia. This has the consequence that the lamp is capable of maintaining an increased temperature for a too long period which may lead to end of life damage.
It is therefore seen to be desirable to develop a discharge lamp in which the arc extinguishing occurs within a time period short enough to prevent the lamp from overheating.
In an exemplary embodiment of the present invention, a discharge lamp comprises a discharge tube having discharge electrodes at both ends thereof. Two lead wires are connected to each of said electrodes that lead to the outside atmosphere from the inside of the discharge tube. The discharge tube contains a fill as an arc discharge generating and sustaining medium. A tungsten coil is connected to at least one lead wire of at least one electrode and placed adjacent to said electrode. At least one material selected from the group consisting of calcium carbonate, barium carbonate and strontium carbonate is applied to the inside and the surface of the tungsten coil.
This structure has the advantage that the discharge arc jumps over from the electrode to the tungsten coil when the emission ability of the electrode significantly decreases. This causes such an abrupt temperature increase in the tungsten coil that the extinguishing gases are released quickly and intensively, and promptly extinguish the discharge. Calcium carbonate, barium carbonate or strontium carbonate or a mixture thereof applied to the tungsten coil are suitable materials for exerting their gas emission capability more efficiently than in other structures of the prior art.